Control of the pharyngeal musculature during wakefulness and sleep: implications in normal controls and sleep apnea

Abstract

Respiration involves the complex coordination of several pump and upper airway/pharyngeal muscles. From a respiratory perspective, the major function of the pharyngeal muscles is to keep the airway patent allowing for airflow in and out of the lung with minimal work by the respiratory pump muscles. The activity of each of the pharyngeal muscles varies depending on its function, but many reduce their activity during sleep. In healthy individuals, these muscles can respond to respiratory stimuli during sleep to prevent airway collapse. However, in individuals with an anatomically small airway, the muscles cannot always compensate for the increased mechanical load. Thus a vulnerable situation in which the airway is prone to collapse may occur with the development of obstructive sleep apnea. This article describes the current understanding regarding the control of the pharyngeal musculature during wakefulness and sleep, as well as the implications for obstructive sleep apnea.

Figure 1. The anatomy and musculature of the human pharyngeal airway

A simplified diagram illustrating several of the important pharyngeal muscles involved in maintaining a patent airway. Panel A is a mid-sagittal view of the human upper airway and illustrates the 4 distinct anatomical subdivisions: the nasopharnyx, retropalatal oropharynx (originally termed velopharynx), retroglossal oropharynx (originally termed oropharynx), and the hypopharynx. Panel B is a parasagittal view. From Kryger et al⁽⁷⁰⁾; reprinted with permission.

Figure 2. Single motor unit discharge patterns in the genioglossus

Examples of the six discharge patterns found in the genioglossus (panel A–F). Each panel shows the raw electromyograph (EMG), the instantaneous discharge frequency plot for the single motor unit and the tidal volume (in litres) for two breaths. From Saboisky et al. (2006) Journal of Neurophysiology, Am Physiol Soc, used with permission⁽¹⁵⁾.

Figure 3. Example of Obstructive Sleep Apnea

An example taken from a clinical polysomnogram in a patient with severe OSA. Note that despite the repeated respiratory efforts to breathe (thoracic and abdominal bands), there is no nasal airflow indicating the airway has become obstructed. Such obstructions are often associated with repeated oxygen desaturations and arousals from sleep. Abbreviations: EEG, electroencephalogram; EMG, electromyogram; EKG, electrocardiogram; SaO₂, arterial blood oxygen saturation.

Figure 4. Determination of Pcrit and its relationship to AHI

A) Illustrative example of how the passive critical pharyngeal closing pressure (Pcrit) is determined. The Pcrit is determined by intermittently lowering CPAP from the subject’s therapeutic pressure to progressively lower levels until zero flow occurred. The peak flow from the 3^rd–5^th flow-limited breaths after a pressure drop are then plotted against mask pressure and are fit with a straight line; the x intercept of this line (zero flow crossing) was taken as the Pcrit. In this example the Pcrit is 0.5cm H₂O. B) Data illustrating the poor relationship between Pcrit and AHI. From Sforza et al⁽⁶⁴⁾; Reprinted with permission of the American Thoracic Society. Copyright © American Thoracic Society.